Japan and the United States on Tuesday marked the 50th anniversary of one of the Cold War's defining security pacts, but an unprecedented level of mistrust between the allies kept celebrations muted.

Japan and the United States on Tuesday marked the 50th anniversary of one of the Cold War's defining security pacts, but an unprecedented level of mistrust between the allies kept celebrations muted.

No major state events were planned in Tokyo to mark the milestone treaty, in which the former World War II adversaries in 1960 stood united against communist Russia and China.

Half a century on, the alliance has been strained since a centre-left government took power in Tokyo four months ago, vowing more "equal" ties with Washington after more than five decades of almost unbroken conservative rule.

The government has signalled a new embrace of Japan's pacifist stance and announced a review of key agreements governing the often-unpopular US military presence in Japan, where 47,000 American troops are now based.

Prime Minister Yukio Hatoyama last week ended a naval refueling mission that has backed US forces in Afghanistan since 2001, and has announced a review of a pact on the relocation of a major US air base.

US President Barack Obama's administration has repeatedly asked Japan's new leaders to stick by the original agreement, under which a new Marine Corps air base would be built on the southern island of Okinawa by 2014.

Despite the row, both sides praised the treaty signed on January 19, 1960, which strengthened a 1951 pact that allowed US forces to be stationed in Japan while providing for the nation's defence under the US nuclear umbrella.

Hatoyama said it was thanks to the pact "that Japan has maintained peace, while respecting freedom and democracy, and enjoyed economic development in that environment since the end of the last World War to this day."

He said he plans to "further deepen the US-Japan Alliance... in order to adapt to the evolving environment of the 21st century. I would like to present the people of Japan with the results of this work before the end of this year."

The anniversary was marked by a naval ceremony with 300 sailors from both countries, led by Japan's naval commander Vice Admiral Masahiko Sugimoto and Rear Admiral Richard Wren, commander of US naval forces in Japan.

But no Japanese politicians or diplomats and no US officials or embassy staff attended the event at Yokosuka port, a US base southwest of Tokyo.

Japan and the United States were due to release a joint statement later in the day, and the US embassy was set to screen a video address by Secretary of State Hillary Clinton on its website, an embassy spokesperson said.

The two countries "can't celebrate this occasion in a big way because it is unclear how Japan wants to change the alliance," said Tetsuro Kato, a political science professor at Hitotsubashi University. "They couldn't say anything more than to confirm that the past 50 years were good."

US Ambassador John Roos, writing in a newspaper, said that although the world had changed since 1960, "the treaty is no less critical today than it was 50 years ago when it was signed."

"Some of the challenges that we face are clear -- North Korea's ballistic missile and nuclear programmes and the lack of transparency in China's military buildup," he wrote in the Asahi Shimbun.

He noted that "Japan benefits from the assistance of US forces in its defence and is able to spend far less on its own defence, as a percentage of GDP, than any other state in the region."

"Given the circumstances, it is extremely regrettable that the Japan-US relationship has become shaky since the launch of the administration of Prime Minister Yukio Hatoyama," Japan's top-selling daily said in an editorial.